[35.06] Lyman-\alpha Emission from Io's Disk and Corona

HST/STIS imaging spectroscopy of Io (Roesler et al. 1999,
Science283, 353) revealed the presence of
strong H Ly\alpha emission from Io's polar regions.
Further analysis (Feldman et al. 2000, GRL27,
1787) suggests that the observed polar emission is primarily
reflected solar Ly\alpha radiation. A strong latitudinal
gradient in the emission is produced by SO2 absorption in
an equatorially confined atmosphere; the inferred column
abundance of SO2 is of order 1016 cm-2 in the
equatorial regions. We examine the expected distribution of
reflected and resonantly scattered solar H Ly\alpha
emission from Io's disk and a neutral hydrogen corona which
surrounds it. Models of the combined emission from Io's
disk, corona and the background contribution from the
interplanetary medium (IPM) are compared with STIS images
obtained between October 1997 and February 2000. Although
the nature of Io's hydrogen corona is uncertain, the Galileo
plasma wave instrument detected emissions attributed to
non-thermal protons during its December 1995 flyby of Io
(Chust et al. 1999, P&SS47, 1377). The density
of pickup protons was estimated at \ge 20 cm-3 at an
altitude of 2 Io radii above the surface. The hydrogen in
the neutral corona is most plausibly supplied by
neutralization of Io plasma torus protons in Io's atmosphere
and exosphere; it is not necessary to invoke an Iogenic
source.